- Received systemic, prescription medications for the treatment of chronic medical conditions, unless such use is on a PRN (as needed) basis only since receipt of Dose 1. Non-PRN use of systemic, over-the-counter medications and PRN systemic, prescription medication may be allowed if, in the opinion of the investigator, they pose no additional risk to subject safety or assessment of immunogenicity/reactogenicity. Note: Topical, nasal, and inhaled medications; vitamins; and contraceptives are also permitted